In their early development neonatal and fetal animals acquire passive immunity as a consequence of the selective transport of maternal immunoglobulin G (IgG) from maternal serum to the circulation of the young. Other immunoglobulins do not traverse the tissue forming the maternofetal surface or the tissue forming the maternofetal interface; the process is specific for IgG. The yolk sac in fowl and rabbits, the gut in many rodents, and the placenta in most other mammals are the primary selective barriers between mother and young. In the chick yolk sac, it is postulated that IgG binds to specific receptors localized to the plasma membrane of the apical surface of the epithelial cells.